What is the primary information that scientists use when comparing fossils with living organisms]?

The primary information that the paleontology used in comparing fossils with living organisms is that every fossil evidence shows that some type of organisms does not or have not changed much over the years. But many of these fossils have also become extinct. It is also show what living things are like in the past and how these change over the time. 

Proteins vitamins and a carbohydrate is a atom